iommu/amd: Consolidate domain allocation/freeing

Merge the allocation code paths of DMA and UNMANAGED domains and
remove code duplication.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
Reviewed-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Link: https://lore.kernel.org/r/20200527115313.7426-7-joro@8bytes.org